There are many different reasons for using backfilling in mines and not only to the benefit of the environment. In some cases, creating an opening underground poses no problem other than with the disposal of waste materials. However, in most types of rock, when an opening is created, it causes stress realignment around the opening. This not only creates problems with spalling and rock falls during rockbursts but also limits the size of the opening that can be made.
Some of the reasons backfill is used in mines (not in any specific order):
(a) to keep highly stressed rock around an opening from spalling. Spalling not only dilutes the ore but it can cause hazards in stopes where people are working; PA1 (b) to keep negatively stressed rock (in tension) around an opening from coming loose for the same reasons in (a) above; PA1 (c) to absorb some of the excess stresses in order to minimize damage from rockbursting; PA1 (d) to act as pillars in some types of mining to permit removal of more ore; PA1 (e) as a working platform for personnel and equipment in undercut and fill operations; PA1 (f) to prevent surface subsidence in shallow mines or soft rock mines; PA1 (g) to alleviate environmental hazards associated with surface disposal of waste materials; and PA1 (h) to dispose of large quantities of mining wastes underground. PA1 (a) cemented rockfill (stiff fill); PA1 (b) uncemented esker sand; PA1 (c) gravel; PA1 (d) uncemented classified mill tailings (cycloned to separate the very fine particles or slimes) and unclassified (total tailings) in hydraulic form (40% to 55% solids content); PA1 (e) cemented classified and unclassified mill tailings in hydraulic form; and PA1 (f) a combination of esker sands and classified and unclassified tailings. With the growing environmental concern for tailings disposal on surface, there has been a growing interest in using the tailings for mine backfill whether or not backfill is necessary for ground control purposes. Mill tailings have traditionally been used for backfilling of mines but usually in a classified form. Once classified, the finer portion of the material (slimes) was then sent to the surface disposal site or tailings pond. However, in recent years, mine operators have seen the need to dispose of their total tailings in the form of backfill.
Various traditional materials have been used for backfill, the choice of which is usually dependent upon the reason for backfilling and on cost. Some of the materials include but are not limited to:
The use of hydraulic tailings fill entails considerable preparation before the filling of the opening can commence. First a bulkhead must be constructed which will hold back the hydraulic mass while allowing water to percolate out of the fill. In some larger mines, drainage pipes such as extruded plastic weeping tile are hung from the top of the stope and brought together under the bulkhead. If a large stope is being filled, a plug is often poured first with a higher cement content (up to 30%) to just above the bulkhead. When the plug is set, the remainder of the stope is filled with the weaker cemented fill (normally around 10% cement). Such extensive preparation is costly in terms of both time and capital.
Another major cost associated with the use of hydraulic backfill is the water that leaches out of the fill mass must be pumped back up to surface. If the mining method used calls for mining to progress against the backfill, sufficient time must be allowed for the fill to drain and cure which can typically be on the order of from 28 to 56 days.
As alternatives to hydraulic fill, high density slurry or paste backfills may be used. High density slurry is simply a thicker hydraulic fill. It has a solids content in the range of 60% to 70% rather than the 40% to 55% range of hydraulic fills. The lower water content requires less extensive bulkhead construction, exhibits faster percolation times, requires less water to be pumped back up to the surface, and requires less cement to achieve the same strength as hydraulic fill.
Paste backfill is an even higher density tailings material in the range of 76% to 84% solids content, depending on the size gradation of the material. By definition, a paste is a material that does not exude water after it has been placed. Such a material has many benefits when used as a backfill in that it does not require water to pumped back up to the surface, lighter or even no bulkhead construction, much less time to resume mining, and it only uses 1.5% to 3% cement. The main drawback in the use of pastefill is that due to its consistency, it is difficult to transport.
Prior Art High Density Slurry / Paste Fill Systems
Currently known systems used to thicken hydraulic tailings slurry to a higher density or paste employ commercially available drum filters, thickeners and/or blend the slurry with alluvial sand to produce paste. While the sand is used in the process to try to achieve a higher strength due to the particle size of the sand, excavation of alluvial sand raises environmental issues.
The thickener system starts out with the tailings in a storage silo. From the storage silo, the tailings are dumped into the thickener. Once the mixture has thickened sufficiently, it is dumped to a mixer where cement is added. After mixing, the resultant backfill material is either pumped or gravity fed to the underground stopes. This thickener system prepares paste in batches as it takes time to remove the water in the thickener. Therefore, if a continuous backfilling system is required, two tailings storage silos and two thickeners are required. The main drawbacks associated with this system are that the thickeners are very costly , i.e. up to $1,000,000 each; being mechanical devices, they require a high level of maintenance; due to the various factors which can affect the thickening process, the fill plant operator must be consistent in judging the release time of the fill from the thickener; thickeners are large and require considerable floor space; and the energy costs to operate such thickeners are relatively high.
The drum filter system begins thickening in the same way, with hydraulic tailings coming from a storage silo. The slurry is fed from the silo to one or more drum filters. These filters use a permeable membrane and air to remove most of the water from the slurry, leaving a cake of tailings (filter cake) on the drums. The actual paste production involves re-pulping the filter cake by adding a controlled amount of water and cement and mixing to obtain the proper consistency. The material is then poured or pumped from the mixer to the stopes.
While the initial cost of a drum filter system is much lower than that of a thickener, the drum filter system still suffers from some major disadvantages which include the large floor space required for the storage of the filter cakes; personnel required to handle the cake between filter, storage and the re-pulping process; the maintenance costs of the filters; and the fact that the operator has less control due to the handling required and the resulting extra personnel involved.